The present invention relates to a laser anemometry probe for optical homodyne detection of frequency offset by Doppler effect.
Mono-axial laser anemometry probes are known that are used to measure aircraft speeds relative to the surrounding environment.
This type of probe is based on a technique consisting in measuring the frequency offset, representative of the relative speed in relation to the air, between a laser beam emitted into the atmosphere and the beam backscattered by the natural aerosols in the air, used as wind field tracers. The expression longitudinal Doppler laser anemometry is used because the frequency offset that results from the Doppler effect is directly proportional to the projection of the speed vector on the line of sight.
The useful information, conveyed by the Doppler frequency offset, is obtained by performing a coherent-type detection; a beam deriving from a coherent light source, for example a laser, is separated into two beams. A first beam called signal beam is sent into the measurement area and a second beam called reference beam or local oscillator constitutes a reference for detection of the Doppler frequency offset.
It is known to use, notably on board aircraft, mono-axial laser anemometry probes, operating at short distances, by means of which a projection of the relative speed vector of the aircraft in relation to the air is obtained on a measurement axis. The local angles of incidence and of sideslip of an aircraft can vary strongly, so it is almost impossible to directly obtain the norm of the relative speed vector in relation to the air.
As illustrated in FIG. 1, a mono-axial laser anemometry probe comprises a laser source SL supplying a linearly polarized reference wave, a separator SEP separating the signal originating from the laser source SL into a signal transmitted to an amplifier AMP and a signal transmitted to a coupler with maintained polarization CPLMP. The mono-axial laser anemometry probe also comprises a device DERF for transmitting/receiving beams in the direction of the axis of the probe, and a backscattered beam separator SFRD arranged between the amplifier AMP and the transmitter/receiver device DERF and transmitting the signal originating from the amplifier AMP to the transmitter/receiver device DERF. The signal backscattered by the transmitter/receiver device DERF is transmitted by the backscattered beam separator SFRD to the coupler with maintained polarization CPLMP. The mono-axial laser anemometry probe furthermore comprises a polarization consistency element EMCP arranged upstream of the coupler with maintained polarization CPLMP in order to ensure that the backscattered signal and the reference wave have the same polarization at the input of the coupler with maintained polarization CPLMP, and a balanced detector DETEQ arranged downstream of the coupler with maintained polarization CPLMP. The coupler with maintained polarization CPLMP supplies on each output pathway half of the backscattered signal and half of the reference wave. The two output signals from the coupler with maintained polarization are transmitted to the balanced detector DETEQ which, for example, comprises two diodes, and delivers as output the difference in the signals supplied by the two diodes. The portion of each signal changing channel in the coupler with maintained polarization CPLMP undergoes a phase delay and the beat produced upon the detection on the two diodes of the balanced detector DETEQ are in phase opposition. The differential output therefore makes it possible to add together the received signal powers. The output signal from the balanced detector DETEQ is then transmitted to an electronic control unit, not represented, to be processed.
The association of two mono-axial laser anemometry probes, with non-parallel axes, makes it possible to obtain the projection of the relative speed vector in relation to the air in a plane defined by these two non-parallel axes. Close to the outer envelope of the aircraft, the relative speed of the aircraft in relation to the air can be only little inclined relative to the outer envelope of the aircraft, which makes it possible, from the projection of the relative speed in a plane containing the two non-parallel axes of the two probes, to determine with sufficient accuracy the relative speed vector in three dimensions of the aircraft in relation to the surrounding air.
The combination of two mono-axial probes forming a biaxial probe is expensive, and the duplication of certain elements increases the risks of failures.
One aim of the invention is to overcome these problems.